1. Field of the Invention
The present invention relates to a directional coupler which is equipped with a main strip line transmitting high-frequency signals, and a sub strip line located in parallel to the main strip line and electromagnetically connected to the main strip line.
2. Background Art
A directional coupler is equipped with a main strip line transmitting high-frequency signals, and a sub strip line located in parallel to the main strip line and electromagnetically connected to the main strip line. The directional coupler uses the electromagnetic connection of the main strip line and the sub strip line to output a part of the high-frequency signals inputted from each terminal of the main strip line to each terminal of the sub strip line.
In the main strip line and the sub strip line, the transmission of high-frequency signals known as even mode or odd mode occurs. The even mode is the case wherein the main strip line and the sub strip line are excited in the identical potential, which is the in-phase equal amplitude. The odd mode is the case wherein the main strip line and the sub strip line are excited in the reverse potential, which is the reversed-phase equal amplitude. The impedance of each mode is determined by the cross-sectional shape of the line.
When the characteristic impedance in the even mode is denoted by Z0e, and the characteristic impedance in the odd mode is denoted by Z0o, the characteristic impedance Z0 of the main strip line and the sub strip line is given by Z0=(Z0e·Z0o)1/2.
By equalizing the phase velocity of each mode, and making the lengths of the main strip line and the sub strip line one-quarter of the wavelength of high-frequency signals, the high-frequency signals inputted from the input terminal of the main strip line appear only at the output terminal of the sub strip line, and favorable isolation characteristics can be obtained. For example, in the case of high-frequency signals of 2.5 GHz, the line length becomes about 30 mm.
A directional coupler attempted loss lowering by connecting the capacitor in parallel to the main strip line, constituting the LC resonating circuit with the main strip line and the capacitor, and resonating the high-frequency signals transmitted to the output terminal of the main strip line from the input terminal of the main strip line, has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 10-290108).